1. Field of the Invention
The present invention relates to a low-noise voltage controlled oscillator with a wide frequency variable range, and more particularly relates to a low-noise voltage controlled oscillator capable of improving a phase noise characteristic over a tuning bandwidth.
2. Description of the Related Art
Conventionally, as a multichannel/multi-access wireless communication method, a phase locked loop (PLL) frequency synthesizer has been widely used from the viewpoint of frequency stability.
A feedback oscillator, for example, is widely used, which can achieve relatively easily a stable output frequency in synchronization with a reference oscillatory frequency.
As the feedback oscillator, a voltage controlled oscillator (VCO) is widely known, which is small in size and operates with low power consumption.
Recent years, since the tendency to convey much information more speedily has been accelerated, a wireless frequency band of a wireless transmission device has been expanded to a frequency band of super high frequency (SHF) or higher, and further has developed to have multiphase and multivalue from the viewpoint of frequency efficiency, so that Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (16QAM), 64QAM, 256QAM, 1024QAM and the like are now being steadily brought into practical use.
Particularly, for various modem methods such as orthogonal frequency division multiplexing (OFDM), since a higher-purity signal source satisfying an error vector magnitude (EVM) characteristic has been increasingly requested to maintain modulation accuracy, an unprecedented low-phase noise characteristic is demanded.
As is well known, as for a higher-purity VCO, a low phase noise VCO is demanded, to which a low noise transistor or a transistor with a low corner frequency and a low cross modulation product is applied, in order to increase an effective load QL of a resonator making up an oscillator.
Regarding these characteristics, a surface acoustic wave (SAW) resonator or a high-dielectric reentrant resonator is generally used for a VCO with a relatively narrow variable frequency range, e.g., a few % or less, for example. However, since a variable frequency of a SAW resonator has a large capacity ratio, it is difficult to obtain a variable range of 1% or more therefrom in general.
Meanwhile, a dielectric resonator is known to have a relatively high Q and is widely applied to a high-purity VCO resonator. A high dielectric reentrant resonator (CAVITY), however, has a feature of reducing an electrical length because of a relatively high dielectric constant (Er) thereof, thus enabling miniaturization, while suppressing a variable range for a general “fixed L and variable C” VCO because characteristic impedance Zo is lowered to 1/√Er, thus inevitably leading to Q deterioration.
In general, from the viewpoint of an increase in a frequency variable range, high characteristic impedance Zo=√(L/C) is required to be set in principle. However, it is widely known that a frequency variable range of a VCO and a low-phase noise characteristic have a tradeoff relationship.
Herein, related prior art includes Japanese Patent Application Laid-Open No. 2007-159114 (Applicant: Synergy Microwave Corporation) (Patent Document 1), Japanese Patent Application Laid-Open No. H05 (1993)-211409 (Applicant: Mitsubishi Electric Corporation) (Patent Document 2), and Japanese Patent Application Laid-Open No. 2004-304667 (Applicant: Toyo Communication Equipment Co., Ltd.) (Patent Document 3).
Patent Document 1 describes a piezoelectric controlled oscillator with multi-octave frequency band selectivity while keeping relatively low and uniform phase noise over a tuning bandwidth.
More specifically, the piezoelectric controlled oscillator is configured so that a multimode coupled resonator is coupled between a base and a collector of a bipolar transistor via a controllable capacitive element and a slow-wave coupled resonator and a progressive-wave coupled resonator are coupled in series with the collector of the transistor.
Patent Document 2 describes a voltage controlled oscillator with low phase noise and a wide tuning range.
More specifically, it describes a piezoelectric controlled oscillator including an oscillator such as a bipolar transistor loaded with two tuned circuits, where the tuned circuits are configured so that a parallel resonance circuit is connected with a series circuit consisting of a varactor diode and an inductor via a line with an electrical length of about 90 degrees in the vicinity of the oscillatory frequency.
Patent Document 3 describes a piezoelectric oscillator achieving a stable oscillatory frequency by stabilizing a base input of an oscillation transistor so as to reduce an influence by fluctuation of a power source.
More specifically, the piezoelectric oscillator includes a parallel circuit of a resistor and a capacity inserted between an emitter of an oscillating amplification transistor and ground and a FET amplifying an output signal of the piezoelectric oscillator, where a collector of the oscillating amplification transistor is connected with a gate of a FET via a capacity and a source of the FET is grounded via a resistor so that a source potential of the FET is applied to a base of the oscillating amplification transistor.
The above-stated conventional piezoelectric controlled oscillators, however, are provided with a simple variable tuned circuit, and a dielectric constant of a dielectric resonator making up a variable tuner thereof is relatively high, for example, about 10 to 90, although it depends on the applied frequency band. Therefore, characteristic impedance Zo is lowered to 0.1 to 0.3 time, so that Zo and an equivalent inductor are significantly reduced, thus causing extreme reduction of QL due to a variable capacity C, which leads to the difficulty to achieve a higher-purity voltage controlled oscillator with a relatively wide frequency variable range.